clang-cl: add the /c, /W0 and /W1 options

This adds a few more clang-cl options. It also exposes two core clang
options to the clang-cl mode: we need to be able to claim --driver_mode
so it doesn't show up as unused in cl mode, and we need -### for tests.

Differential Revision: http://llvm-reviews.chandlerc.com/D1232